1. Field of the Invention
The present invention relates to a spindle device and particularly, to a spindle device capable of detecting the abnormality of an antifriction bearing which rotatably supports a spindle.
2. Discussion of the Related Art
Heretofore, as methods of detecting the abnormality of an antifriction bearing, there has been known one which is described in, for example, JP2008-064183 A (hereafter referred to as “Japanese application”). In a method described in the Japanese application, the temperature of an antifriction bearing is detected, and when the detected temperature exceeds a threshold value, it is judged that the antifriction bearing is abnormal.
However, the temperature of such an antifriction bearing changes in dependence on the rotational speed of the spindle. Thus, in the prior art, the threshold value has been set in relation to the maximum rotational speed of the spindle. For example, spindles for machine tools have been used at various rotational speeds meeting objectives of machining. Specifically, where a spindle of a machine tool is designed to have the maximum rotational speed of 20,000 rpm (revolutions per minute), a rotational speed of, e.g., 5,000 rpm is used at a certain machining step, and another rotational speed of, e.g., 10,000 rpm is used at another machining step.
In such state of use, there is exhibited a behavior that the temperature of the antifriction bearing rises right after the rotational speed is increased and then, reaches a saturated state. That is, when the rotational speed of the antifriction bearing is increased from 0 rpm to 5,000 rpm, the temperature of the antifriction bearing rises for a short time and becomes approximately stable at a temperature corresponding to the rotational speed of 5,000 rpm. Then, when the rotational speed is increased from 5,000 rpm to 10,000 rpm, the temperature rises for a short time and becomes approximately stable at another temperature corresponding to the rotational speed of 10,000 rpm.
In the method described in the Japanese application, the temperature threshold value is a threshold value set in relation to the maximum rotational speed, and therefore, where the spindle device is used at a rotational speed which is considerably lower than the maximum rotational speed, the temperature difference between the actual temperature of the antifriction bearing and the threshold value becomes very large. Therefore, there is a possibility that when an abnormality occurs in the antifriction bearing rotating at such a considerably lower speed, the antifriction bearing is overheated to seizure before the temperature of the antifriction bearing reaches the temperature threshold value.